I have a 5D2 & 5D3 with 1 580EX2 Flash. I'm using the flash with a set of 1 Mini TT1 & Flex TT5 on either body. I'm looking to purchase a new 600EX-RT however, I'm not sure if I just need to buy 1 Mini TT1 or both the Mini TT1 and Flex TT5 PW?

I know the 600EX is wireless, so, can I get away with having a PW Mini TT1 on my 5D3 to trigger the flash?

What in your opinion is the best way to do this set-up so I can trigger the flash off camera?

I doubt the frequencies are the same. I do know this. You can put a transmitter into a Sekonic flash meter that will work with PW and fire a flash when a PW receiver/transmitter is attached to the flash. Sorry don't know what the mini or flex actually do so I just called it receiver/transmitter. The Sekonic won't fire the 600.

I contacted Sekonic to see if they were ever planning to develop a transmitter that would fire a 600 and they said no. They are partners with PW. So if the Sekonic and PW are on their own frequencies as opposed to Canons then as I said I doubt it will work.

PW and Canon RT can't trigger each other. Different freqs, different protocols.

You'd need the 600EX-RT on a Flex TT5 to trigger it. Or, go all Canon with an ST-E3-RT or a second 600 on the camera to trigger the 600 (but the 580 can't be radio triggered without a 3rd party trigger, or at all by the ST-E3).

PW and Canon RT can't trigger each other. Different freqs, different protocols.

You'd need the 600EX-RT on a Flex TT5 to trigger it. Or, go all Canon with an ST-E3-RT or a second 600 on the camera to trigger the 600 (but the 580 can't be radio triggered without a 3rd party trigger, or at all by the ST-E3).

Got it neuroanatomist! So I would need to buy an additional set of PW's, a Mini TT1 on camera body and a Flex TT5 on the 600EX-RT??

You need one Mini TT1 on the body you're using. If you use two bodies during a shoot, it may make sense to have a second Mini TT1. A single Mini TT1 transmitter can control a whole bunch of receivers/tranceivers. I use an on-camera Mini TT1 to control a pair of Flex TT5s and a PowerMC2 for an Einstein monolight (and it's the ability to trigger that monolight that's holding me back from Canon's RT system).

The Flex TT5 can be either a receiver or a transmitter, so you could have a Mini on one camera and a Flex on the other camera with one flash, or a Mini on one camera and a Flex on each of two flashes.

The Mini/Flex system allows E-TTL automatic flash metering, the Plus III system does not.